Use of honeycomb structural materials is becoming increasingly widespread. Applications include such diverse fields as aerospace manufacturing, cushioning heels for sports shoes, and furniture construction. Most of these uses rely on the high rigidity and load bearing capacity created when honeycomb material is sandwiched between suitable facing sheets.
Honeycomb materials may be divided into several categories depending upon such factors as their material of construction and mode of preparation. Materials of construction include paper, woven and non-woven glass and carbon fiber reinforced thermoplastics and thermosetting resins, thermoplastic films and woven materials, light metals such as aluminum, and steel.
Honeycomb may be roughly divided into expanded and non-expanded types. In expanded honeycomb, to which this invention does not apply, a "stack" is made of planar material, each successive sheet of material being bonded to the previously laid-down sheet along spaced, parallel bond lines. The bond lines between the sheet and those between the two previous sheets are staggered. The bonds may be a simple phenolic or other adhesive, as in the case of paper honeycomb, epoxy or other high performance adhesive in the case of fiber-reinforced material, or the bonding may be accomplished by thermal fusing or welding of thermoplastic and metal honeycomb, respectively. Following production of the stack, it is secured at the top and bottom surfaces and pulled apart to produce the honeycomb structure. In the case of paper honeycomb and the like, the stack may be fixed in its expanded form by being immersed in a thermosetting resin bath and the resin impregnated paper honeycomb cured while expanded. In the case of thermoplastic honeycomb, the core may be exposed to a sufficiently elevated temperature to remove any "memory" of the unexpanded condition, again fixing the core in the expanded condition.
For example, U.S. Pat. No. 5,277,732 discloses a process for the automated preparation of aluminum foil honeycomb where strips of aluminum foil having parallel, spaced apart adhesive strips, are indexed at the proper locations by sensing devices, the adhesive cured, and the stack expanded. In U.S. Pat. No. 4,957,577, thermoplastic honeycomb consolidated by thermal fusion is prepared in a process wherein metallic release substrates are assembled with a stack of thermoplastic strips at positions such that the heat required for thermal fusion of the topmost two strips along spaced bond lines to produce the necessary node-antinode bonds does not penetrate and fuse the second and third strips to each other. Following expansion of the core, the release substrates are removed. In U.S. Pat. No. 5,421,935, an improvement in the process of the '577 patent is achieved through elimination of the metallic release substrates. In the '935 patent, thermal fusion of more than just the top two stack layers along the node-antinode bond lines is prevented by carefully adjusting the temperature and duration of the fusion such that the antinodes in the topmost sheet are completely fused, but the penultimate sheet nodes directly below the topmost sheet antinodes are only fused to approximately 75% of the thickness of the thermoplastic.
The processes described above are not suited for a variety of high performance honeycomb materials for numerous reasons. Many desired materials of construction cannot be used with expanded honeycomb. Such materials include fiberglass and particularly carbon-reinforced polymer systems, where stack expansion results in wholesale fiber breakage or node bond breakage. The loss in strength offsets much of the performance advantage expected of these materials. Closely related is the fact that the expansion process creates stress at numerous points in the structure which affect the performance of even non-reinforced materials such as thermoplastics. Annealing thermoplastic cores may alleviate some of these stresses, however at the expense of increased process time and the risk of core distortion. The process of the '577 patent, with its many, thin, release substrates, does not lend itself to automated production, and the process of the '935 patent is highly sensitive to material and process parameters.
Non-expanded honeycomb is specified for demanding applications, particularly those which require the use of fiber-reinforced materials, whether due to their unique physical properties or electromagnetic properties such as may be required in so-called "stealth" products. However, non-expanded honeycomb of such materials has been notoriously difficult and timeconsuming to manufacture.
For example, U.S. Pat. No. 3,356,555 discloses a method of preparing non-expanded honeycomb in which a fiber-reinforced polymer web is corrugated through the use of a squirrel cage roller and laid atop a bed of hexagonal bars. A second layer of hexagonal bars are placed in the antinodes of the corrugated web, and a second corrugated web, whose nodes are displaced from those of the first layer, positioned atop these bars such that the nodes of the first web are proximate the antinodes of the second web. This procedure is repeated with alternating layers of hexagonal bars and corrugated web until the desired core height is reached, following which the stack is pressurized between caul plates at the top and bottom of the stack and heated to fuse or cure the polymer system where the nodes and antinodes of successive layers abut. The hexagonal bars are then removed from the core by pulling them out or pushing with a thin pushrod. Great Britain Patent No. GB-A-2 188 166 discloses a similar process, as does U.S. Pat. No. 5,131,970.
The processes of the '555, '970 and like patents present many problems which prevent their widespread use. First, the many hexagonal bars which must be removed from the core following consolidation are difficult to remove without distorting or destroying the core. The thermoplastic and thermosetting polymers may stick to the bars, and even the application of release agents such as silicones is often not sufficient to eliminate this problem.
A further drawback is associated with the geometry of the rod/web layup. As the node-antinode demes are of double thickness due to the abutting of the two webs to be bonded, while the remaining honeycomb cell walls are only of single thickness, for proper consolidation, the hexagonal bars must be other than symmetrical in cross-section. Even when the proper geometries are achieved, a mere change in web thickness may require a different set of bars.
Consolidation of such corrugated stacks is also problematic, as it is difficult to achieve uniform consolidation without resorting to high pressure between caul plates. If too high a pressure is used, resin may be forced out of the node-antinode demes resulting in resin-starved areas not having optimal strength properties.
However, the greatest drawbacks to the use of batch core processes such as those described is the labor-intensive and part-count intensive preparation. For example, a four foot (1.2 m) length of honeycomb having a 0.125 inch (3.2 mm) cell width, and being only four cells in height, would require in excess of 1400 metal bars. The manual operations involved in assembling the stack result in extremely long fabrication times. It is not unusual, for example, for greater than 24 hours to be involved with the preparation of one cubic foot of honeycomb. Furthermore, the use of precorrugated sheets renders layup difficult. The corrugated material must be laid up such the antinodes of the top layer abut the nodes of the previous layer. However, corrugated sheets tend to nest instead, and thus application of each new layer involves a considerable amount of time.
To avoid the aforementioned extended preparation time, U.S. Pat. No. 5,296,280 discloses a method for preparation of adhesively bonded, non-expanded honeycomb. In the '280 patent, webs are corrugated and the nodes are antinodes of the respective sides coated with one part of a two-part cyanate resin/epoxy resin adhesive. The abutting nodes/corrugated webs are positioned such that the nodes and antinodes abut, and the stack, once prepared, cured at elevated temperature. The process disclosed in the '280 patent suffers from the necessity to use a tacky, thermosetting adhesive, which will not always provide the chemical and physical properties desired. The necessity for oven cure of the stack is also a drawback, not only due to the extra time and expense involved, but also due to the potential for core distortion during cure.
U.S. Pat. No. 5,354,394 purports to produce non-expanded honeycomb without prior corrugation of the honeycomb material. Materials disclosed for use include thermoplastic and fiber-reinforced thermoplastic webs. A stack of heating rods and mandrels is assembled with the web material intermediate to staggered layers of heated rods and the assembly compressed between caul plates. This method may be suitable for use with non-reinforced materials, but it is difficult to conceive how fiber-reinforced materials can be used, since the dimensions of the web after corrugation are considerably less than the linear length of the non-corrugated web. While non-reinforced thermoplastics may distort and flow to accommodate the difference in length transverse to the corrugations of the corrugated material, fiber-reinforced materials cannot do so.
In U.S. Pat. No. 5,139,596, the present inventor disclosed a process for the continuous preparation of honeycomb material by the repetitive forming of honeycomb one-half cell height at a time. In contrast to batch methods, the process of the '596 patent allows continuous production of honeycomb from a variety of materials. The process of the '596 patent involves the use of but two sets of retractable, displaceable former bars, one set being disposed within the topmost row of complete honeycomb cells, the second set being disposed in the antinode depressions of the top face of the honeycomb. A corrugating roller corrugates and bonds a polymer-containing web simultaneously as it traverses across the length of the honeycomb in a direction transverse to the axis of the corrugations. The heat source is a non-contacting heat source such as a hot shoe out of which flows a stream of hot gas. The gas stream is directed toward the bottom of the uncorrugated web and the top of the honeycomb. The web, softened by the heat, is corrugated and immediately fused to the top of the honeycomb by the roller which presses the heated web and core antinodes and nodes together. The lower-most set of formers are then retracted and positioned on the top of the core, and the process repeated.
The process of '596 has numerous advantages over prior processes. Only two sets of former bars are required, their number and size dictated by the width and length of the honeycomb, and not by its height. The process is also rapid, does not require a separate adhesive, and does not require an oven cure. The process has the drawback, however, of permitting only a limited time of pressurized contact between the node-antinode demes. As the heat source is a non-contacting heat source, the heat supplied must often be excessive in order that sufficient heat remain to fuse the node-antinode surface. Production of honeycomb cores from thin, woven thermoplastic webs and thermosetting webs are problematic, the former due to melt through at the node-antinode demes or during corrugation, the latter due to insufficient consolidation time to cure the thermoset resin.
U.S. Pat. No. 5,399,221 discloses a process using an apparatus in some ways similar to that of the '596 patent in that the use of a pair of retractable former bars are used. However, in the '221 patent, the web is precorrugated and indexed over the rods, following which a heated platen presses down from above, fusing the node-antinode demes. An advantage of the '221 patent is that the temperature required for corrugation is completely independent of the consolidation temperature. A further advantage is that the platen may exert pressure over a longer time period and may also be alternatively heated and cooled. However, the '221 process has the disadvantage that fully corrugated sheets must be accurately indexed, an operation known to those skilled in the art to be difficult to perform in a reproducible manner. A further disadvantage is the difficulty of maintaining uniform pressure of the platen against the surfaces to be bonded across the length and width of the core. Thus, some portions of the core will be bonded more fully than others, producing a non-uniform material. This disadvantage is particularly important for large core widths.
It would be desirable to provide a non-batch method for the continuous preparation of non-expanded honeycomb core having uniform properties. It would be further desirable to provide a process which can successfully employ many core materials such as paper, fiber-reinforced thermoplastic and thermosetting webs, non-reinforced thermoplastics, and metals, these with or without added heat-curable adhesives, while providing a strong node-antinode bond. It would further be desirable to provide such a method where the corrugation temperature and bonding or consolidation temperature are substantially independent, and where each node-antinode may be selectively subjected to different and controllable temperature cycles over extended periods of time. It would further be desirable to provide a continuous process which does not employ precorrugation.